Optimal galaxy survey for detecting the dipole in the cross-correlation with 21 cm Intensity Mapping

TL;DR

This paper explores how to optimize galaxy surveys for detecting the relativistic dipole in the cross-correlation with 21 cm intensity mapping, proposing strategies to maximize signal-to-noise ratio.

Contribution

It introduces an optimal galaxy catalog selection method and analyzes the impact of biases and magnitude thresholds on the dipole detection.

Findings

Signal-to-noise does not increase with bias difference due to shot noise.

Optimal magnitude threshold enhances the dipole detection.

Potential to increase detection SNR by up to a factor of five.

Abstract

We investigate the future perspectives of the detection of the relativistic dipole by cross-correlating the 21 cm emission in Intensity Mapping (IM) and galaxy surveys at low redshift. We model the neutral hydrogen (HI) and the galaxy population by means of the halo model to relate the parameters that affect the dipole signal such as the biases of the two tracers and the Poissonian noise. We investigate the behavior of the signal-to-noise as a function of the galaxy and magnification biases, for two fixed models of the neutral hydrogen. In both cases we found that the signal-to-noise does not grow by increasing the difference between the biases of the two tracers, due to the larger shot-noise yields by highly biased tracers. We also study and provide an optimal luminosity-threshold galaxy catalogue to enhance the signal-to-noise ratio of the relativistic dipole. Interestingly, we show that the maximum magnitude provided by the survey does not lead to the maximum signal-to-noise for detecting relativistic effects and we predict the optimal value for the limiting magnitude. Our work suggests that an optimal analysis could increase the signal-to-noise ratio up to a factor five compared to a standard one.